The present invention relates to an electrostatic chuck useful for holding a substrate.
Electrostatic chucks are used to hold semiconductor silicon wafers during processing in a process chamber. A typical electrostatic chuck comprises an electrode covered by a dielectric layer. The electrode of the chuck is electrically biased with respect to the substrate by an applied voltage. Process gas is introduced into the process chamber, and in certain processes, an electrically charged plasma is formed from the process gas. In monopolar electrode chucks, the electrical voltage and plasma induce opposing electrostatic charge in the chuck and substrate that result in an attractive electrostatic force that electrostatically holds the substrate to the chuck. In bipolar electrode chucks, the bipolar electrodes are electrically biased relative to one another to provide an electrostatic force that holds the substrate to the chuck.
The electrostatic attractive force generated by electrostatic chucks 10a, 10b can be of two different types. As schematically illustrated in FIG. 1(a), a dielectric layer 11 with a high resistance provides Coulombic electrostatic forces that are generated by the accumulation of electrostatic charge in the substrate 12 and in the electrode 13 of the chuck 10a, causing polarization within the dielectric layer 11. The Coulombic electrostatic pressure is described by the equation: ##EQU1## where .epsilon..sub.0 and .epsilon..sub.r are the dielectric constant of vacuum and relative dielectric constant of the dielectric layer 11 respectively, V is the voltage applied to the electrode 13, t is the thickness of the dielectric layer, and .delta. denotes the contact resistance of an air gap between the substrate 12 and the dielectric layer 11. The electrostatic force increases as the relative dielectric constant .epsilon..sub.r of the dielectric layer 11 increases.
With reference to FIG. 1b, Johnsen-Rahbek or non-coulombic electrostatic attraction forces arise when an air gap interface 14 between a low resistance dielectric layer 15 and the substrate 12 has an interfacial contact resistance much greater than the resistance of the dielectric layer 15, i.e., when the resistance of the dielectric layer 15 is typically less than about 10.sup.14 .OMEGA. cm. In these chucks, electrostatic charge drifts through the dielectric layer 15 under the influence of the electric field and accumulates at the interface of the dielectric layer 15 and the substrate 12, as schematically illustrated in FIG. 1(b). The charge accumulated at the interface generates an electrostatic pressure represented by the equation: ##EQU2## where .delta. denotes the contact resistance of the air gap between the substrate 12 and the low resistance dielectric layer 15. The Johnsen-Rahbek electrostatic attractive force is larger for a given applied voltage because polarization in the dielectric layer and free charges accumulated at the interface (which is at a small distance from the substrate) combine to enhance electrostatic force. A strong electrostatic force securely clamps the substrate 12 onto the chuck and improves thermal transfer rates. Also, it is desirable to operate the chuck using lower voltages to reduce charge-up damage to active devices on the substrate 12.
It is known to use ceramic formulations to fabricate the low resistance dielectric layers 15 to provide Johnsen-Rahbek electrostatic chucks. For example, Watanabe et al., disclose various formulations of Al.sub.2 O.sub.3 doped with low levels of 1-3 wt % TiO.sub.2 to form low resistance ceramic dielectric layers, in "Relationship between Electrical Resistivity and Electrostatic Force of Alumina Electrostatic Chuck," Jpn. J. Appl. Phys., Vol. 32, Part 1, No. 2, 1993; and "Resistivity and Microstructure of Alumina Ceramics Added with TiO.sub.2 Fired in Reducing Atmosphere," J. of the Am. Cer. Soc. of Japan Intl. Edition, Vol. 101, No. 10, pp. 1107-1114 (July 1993). In another example, U.S. Pat. No. 4,480,284 discloses a chuck having a ceramic dielectric layer made by flame spraying Al.sub.2 O.sub.3, TiO.sub.2, or BaTiO.sub.3 over an electrode, and impregnating the pores of the ceramic dielectric with a polymer. Whereas pure Al.sub.2 O.sub.3 ceramic layers have resistivities on the order of 1.times.10.sup.14 ohm cm, the alumina/(1-3 wt % titania) ceramic formulations typically have resistivities on the order of 1.times.10.sup.11 to 1.times.10.sup.13 that provide Johnsen-Rahbek electrostatic forces.
However, one problem with such conventional ceramic dielectric layers, arises from their poor dechucking properties, which refer to the degree of difficulty of removing the substrate from the surface of the electrostatic chuck on completion of processing. The electrostatic charge which accumulates in the substrate and below the surface of the dielectric layer, does not dissipate even after the voltage applied to the electrode is turned off. For example, Nakasuji, et al., in "Low Voltage and High Speed Operating Electrostatic Wafer Chuck Using Sputtered Tantalum Oxide Membrane," J. Vac. Sci. Technol. A, 125(5), Sep/Oct 1994, describes an electrostatic chuck coated with Al.sub.2 O.sub.3 doped with 1 wt % TiO.sub.2, that takes from 20 to 120 seconds for the residual electrostatic charge to dissipate. As a result, the substrate cannot be removed from the chuck until the accumulated charge gradually dissipates. The time delay for charge dissipation provides unacceptably low processing throughput.
Various methods have been used to dissipate or neutralize the accumulated electrostatic charge, including (i) use of ionized gases to neutralize the charge, (ii) oscillating sinusoidal AC voltages to prevent formation of the electrostatic charge, or (iii) providing high transient currents at the transitions of the waveforms of the AC voltages applied to the electrode. However, these solutions require additional processing steps and can cause currents that damage active devices on the semiconductor substrate. Sinusoidal AC waveforms can result in resonance vibrations that damage monocrystalline silicon substrates and cause movement of the substrate during processing. Also, oscillating AC voltages remove the surface charge at a rate which typically results in about a 2-to-4 second dechucking release time. This is slow in relation to the required speed of the robotic handling devices that are used to pick-up and drop-off substrates onto the electrostatic chuck. Any residual force that remains due to incomplete discharge of the residual charge can cause the substrate to break or slide in unpredictable directions during pick-up or drop-off of the wafer by a robotic arm. One must either wait for the electrostatic clamping force to decay or apply a potentially damaging lifting force to remove the substrate.
Thus there is a need for an electrostatic chuck that provides quick charging and discharging response times, to allow rapid chucking and dechucking of the substrate held on the chuck. There is also a need for a discharging chuck that will not cause excessive leakage of electrostatic charge during operation of the chuck or dissipate the electrostatic holding force during processing of a substrate. It is further desirable for the electrostatic chuck to avoid discharging a surrounding plasma during processing of the substrate.